Bronchiolitis: Inflammation of the bronchioles, the airways that extend beyond the bronchi and terminate in the alveoli. Bronchiolitis is due to viral infections such as parainfluenza, influenza, adenovirus and, especially, respiratory syncytial virus (RSV).

Bronchiolitis is largely a disease of infancy. It starts as a mild upper respiratory infection and then worsens. There is increasing respiratory distress and wheezing. The baby's breathing rate increases markedly (a condition called tachypnea). The nostrils flare with each breath and the muscles between the ribs retract as the child breathes. It is an emergency if the infant become grey or bluish (cyanotic).

Bronchiolitis is more frequent in the late fall, winter, and earlty spring. More than half of all infants have been exposed to RSV by their first birthday. Many have few or no symptoms. Some become very ill with bronchiolitis.